Variously, signal presence detectors, optical channel power monitors, etc. are used to detect the presence or absence of a wavelength in optical protection switching systems. For example, optical 1+1 path protection schemes are used to protect communication links from path failures (e.g., fiber cuts) since fiber cuts are significantly more likely than transceiver failures. In a 1+1 path protection scheme, a signal originating from a single transmitter is transmitted to a single receiver over two independent paths, preferably diverse from one another. At the receiver's location, for each of the two terminating paths, the signal of interest is monitored by a signal presence (SP) monitor and a 2×1 optical switch selects one of the two paths based upon signal presence monitoring. If the selected input's signal is absent, and the alternate signal is present, the switch will select the alternate signal. A common implementation for 1+1 path protection of an optical communication link is to use a 50:50 power splitter to duplicate the transmitted signal for transmission over the two independent paths, and a 2×1 optical switch with signal presence detectors at each input to select an active path.
When a communication link is bidirectional, each end of the link has a transmitter and a receiver and providing 1+1 path protection requires placement of a 50:50 power splitter and a 2×1 switch with signal presence detectors at each end of the link. A common hardware implementation is an “Optical Protection Switch” (OPS). Conventionally, an optical filter precedes the OPS switch, affording two simplifications: (1) the OPS's signal presence monitors can be signal taps with integrated power monitors, providing a simple and low-cost solution; and (2) there are no interfering signals present at the receiver, and therefore, no additional filtering required to receive the signal of interest.
Receivers have started using coherent detection techniques, which relaxes or eliminates the need for an optical filter to precede the receiver. Specifically, coherent detection systems typically receive a fiber with a plurality of wavelengths and tune to the wavelength of interest, i.e., the connection to a receiver may contain interfering wavelengths. Interfering wavelengths are rejected and filtered out within the coherent receiver. Specifically, it is not possible to simply monitor total integrated power to detect the loss of a wavelength of interest since it would not indicate which specific wavelength or wavelengths have been changed. A conventional signal presence detector that performs in a multi-signal environment include (1) an optical tap followed by a tunable bandpass filter followed by a photodetector, with the tunable filter set to the wavelength being protected by the 1+1 scheme; (2) an optical tap followed by a scanning filter followed by a photodetector; and (3) an optical tap followed by a wavelength demultiplexer, each demultiplexed port terminating at a unique photodetector. Disadvantageously, these solutions are expensive, requiring a spectrometer for each of the signal presence monitors, or a single spectrometer that is time-shared between the two switch inputs. Another approach may include installing a communication port at the OPS where an external input (e.g., network management system, control plane, Software Defined Networking (SDN) controller, etc.) can relay port status. This approach requires additional processing in the OPS and a communication path for control of the OPS.
In order to extend 1+1 path protection using an OPS to links using coherent receivers with wavelength demultiplexers that do not optically filter wavelengths, it is, therefore, desirable to develop a single wavelength presence monitor that performs in a multi-wavelength environment.